Increasing prevalence of avian malaria in species such as the great tit has paralleled the rise in global temperature. Photo credit: Sebastian Hoppe, Creative Commons Attribution ShareAlike 2.5 Generic

As Earth’s temperate zones warm, it seems inevitable that mosquitoes from the tropics and subtropics—and the disease-causing parasites they carry—will spread. Mosquito-borne diseases like dengue and malaria are of particular concern, although the impact of climate change on their spread remains unknown, in large part because climate models currently are incapable of accounting for health care and related factors. But if a recent study of malaria in birds published in the May issue of the journal Global Change Biology is any indication, climate change may in fact be associated with the spread of mosquito-borne disease.

“The most doubtful issue associated with human malaria is that it is difficult to know whether the change in parasite distribution in time is due to the effect of climate change or is mediated by socio-ecological factors,” explained László Z. Garamszegi, author of the study and a researcher at the Doñana Biological Station in Seville, Spain.

But having compiled data on avian malaria from previous surveillance studies relying on microscopic screening of blood, Garamszegi saw an opportunity. “I realized that [the bird data] was almost free of those socio-ecological confounders that blur our understanding about the spread of human-infecting malaria in recent years.”

Garamszegi analyzed 43 surveys covering a time span from the 1940s to the present. Together, the surveys provided data on more than 3,000 bird species that had been screened for malaria in different regions. For each species in the study sample, Garamszegi calculated prevalence, and “Since each study was done in a given period and locality, I could assign climate data to each,” he said. “If the same species was screened in another study in another year and locality, I could compare the climatic and temporal data for the same species over [multiple] studies.”

What he found was surprising—avian malaria has nearly tripled in the last 70 years, in parallel with increasing global temperatures. Notable among the species of birds most heavily affected were house sparrows, great tits, and blackcaps. Prior to 1990, when global temperatures were cooler than in recent years, less than 10 percent of house sparrows (Passer domesticus) were infected with malaria. In recent years, however, this figure has increased to nearly 30 percent. Likewise, since 1995, the percent of malaria-infected great tits (Parus major) has risen from 3 to 15. A 1999 study Garamszegi included in his analysis revealed that some 4 percent of blackcaps (Sylvia atricapilla)—a species once unaffected by avian malaria—were infected.

He also found that Africa and Europe, regions connected by major bird migratory routes, experienced the most obvious increases in prevalence. In contrast, increases in prevalence were less common in North and South America and Asia.

Whether the increase in avian malaria reflects a wider climate change-related distribution of the parasite or parasite-carrying mosquitoes remains unclear. “Mosquitoes are harder to screen,” Garamszegi explained. “But indeed there is evidence that some mosquito species are showing up in new places.”

Although Garamszegi’s findings are compelling, they remain correlational, as it is currently impossible to study experimentally the effect of climate change in natural systems. “We should consider other factors that can also increase year by year and that could potentially mediate disease susceptibility in birds,” he said. “For example, habitat may also be deteriorating for various reasons, [such as] industrial-scale agriculture and pollution. These factors could also mediate changes in infection status.”

To gain a better understanding of the threat of avian malaria, Garamszegi suspects that it would be worthwhile to track the long-term prevalence of the disease in the same populations and in populations that are not affected by habitat destruction. “This is what we are now trying to do in an international collaboration,” he said. “It would also be informative to determine what factors affect the reaction of different species to global warming in terms of parasite load and to monitor mosquito populations in a standard way.”

Until more is known about these factors, controlling the spread of avian malaria will be extraordinarily difficult. As for a solution to this complex situation, Garamszegi added, “It is hard to say something, except ‘stop global warming‘.”

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